164 Transactions of tee American Institute. 



must therefore be gases or vapors ; and by carefully studying tliese 

 bands, and comparing their position with the bands of the metallic 

 elements with which we are acquainted, we hope to reach some 

 knowledge in regard to the constitution of these nebulse and the 

 nature of the gases of which they consist. They generally show us 

 three distinct lines, and one of these lines has precisely the same 

 position as one of the bands of the hydrogen spectrum. But then 

 the hydrogen spectrum has itself three distinct bands — a red band, a 

 green band, and a blue band. The green band corresponds with 

 one of the bands seen in tlie nebulte ; but the red band is fully as 

 bright as the green, and we find no corresponding red band in the 

 spectrum of the nebulae. Hence we have not yet been able to reach 

 any positive conclusion in regard to the nature of the gas of which 

 the nebulae consist ; but so far as these experiments go we should 

 conclude that the nebulae must be composed of a more elementary 

 kind of matter than any with which we are acquainted upon the sur- 

 face of the globe. 



There remains one other general principle in connection with this 

 subject which demands our attention before closing the lecture. 

 This principle may be deduced from the well known theory of light ; 

 but we shall only have time to consider it as an established fact. 

 The principle is this : Every gas or vapor eagerly absorbs the light 

 which it emits itself. Thus, for example, this vapor of sodium, which 

 is giving us a pure yellow light, equally absorbs that same yellow 

 light. Hence, if we put behind this ilame another source of yellow 

 light, then that yellow light would be wholly cut off by the sodium 

 flame. The sodium flame is just as opaque to yellow light of the 

 same quality as a thick deal board ; but, what is very remarkable, it is 

 perfectly transparent to light of every other color. Hence, if we 

 place in this second flame a bit of lithium, the light of the lithium 

 will pass through the sodium flame, and the result will be a mixed 

 effect produced by the light of the sodium plus the light of the 

 lithium. Ton will not see, of course, pure red, but you will see red 

 mixed with yellow ; and the yellow being far more intense, the yel- 

 low will necessarily be the predominant tint. Taking the bit of 

 sodium partially from the flame, to reduce its intensity, and make 

 the two more equal, you see the effect produced in a more marked 

 manner. Suppose, now, we place behind the sodium flame, not a 

 lithium flame, but a white flame, what then will be the result ? That 

 ."white flame gives us light of every tint and color ; and, therefore, it 



